1. Field of the Invention
The present invention relates to a method and apparatus for polishing, and more particularly, to a method and apparatus for polishing a wafer using Chemical Mechanical Polishing (CMP).
2. Description of the Related Art
In recent years, the advance in semiconductor technologies has promoted finer design rules and multilayer wiring structures, and wafers have become larger in attempts to reduce costs. Such finer design rules have increasingly reduced the depth of focus of a stepper in a photolithography process, resulting in a difficulty to precisely provide a specified wiring width due to small roughness on a wafer surface.
Surface planarization process for each wiring layer has therefore been practiced. A Chemical Mechanical Polishing (CMP) apparatus is used in the planarization process. The apparatus dispenses slurry that contains fine abrasive grains and chemicals, while pressing a wafer surface to be planarized against a rotating polishing pad, and polishes the wafer with a combined effect of chemical and mechanical effects. The apparatus has been a candidate in recent years particularly for planarizing metal layers such as Cu wiring, W plug and the like. For the CMP process removing Cu layers, an electrochemical mechanical polishing apparatus is also proposed, which applies voltage for polishing between a work to be polished, i.e. a wafer having Cu layer thereon, and an polishing platen in order to improve the removing efficiency in polishing, reduce surface roughness, etc.
Such wafers having electrically conductive layers such as Cu and W to be polished thereon, however, have an extremely active surface, which leads to inconvenience in polishing due to a surface oxidation during a polishing process. In particular, when Cu, for example, is selectively removed by electropolishing, the electrical conductivity of the Cu surface has a significant effect on the polishing rate. An oxide layer formed on the Cu surface greatly reduces the conductivity and compromises the polishing rate that would correspond to the applied voltage. This has presented difficulty in securing a constant polishing rate.
Oxidized Cu surfaces also alter the surface hardness relative to unoxidized surface, causing a change in the mechanical strength, and thus the polishing rate. A surface oxidation on a metal layer that causes a change in the mechanical strength as well as the conductivity, therefore, presents problems that a constant polishing rate cannot be secured in a CMP apparatus using an electrolytic effect.